Orthogonal frequency division multiplexing (OFDM) in which a plurality of subcarriers can be effectively used has been recently studied for a large number of radio communication systems. An OFDM radio resource is indicated with a plurality of subcarriers and an OFDM symbol indicative of a communication unit time.
A base station of a radio communication system that employs OFDM assigns a subcarrier of an OFDM symbol to data for a mobile device. A computation method for assigning a subcarrier to the data includes, for example, MAX-CI and proportional fair. The base station optimally assigns a subcarrier for each mobile device in consideration of a reception environment for each mobile device to realize an effective use of a plurality of subcarriers.
Moreover, the base station performs a control (hereinafter, “AMC (adaptive modulation and coding) control”) for dynamically changing a modulation method of data that is used for transmitting data for a mobile device, an encoding ratio of an FEC (forward error correction) code, and the like. For example, the base station determines a combination (hereinafter, “MCS (modulation and coding scheme)”) of the modulation method and encoding ratio of data in accordance with a reception environment for each mobile device in order to perform optimum data transmission.
In a technique for performing the assignment of a subcarrier and the determination of MCS, it is important that a base station correctly grasps a reception environment for each mobile device to realize preferable performance. In order that the base station grasps a reception environment for each mobile device, it is generally known that a mobile device feeds back a channel quality indicator (hereinafter, “CQI”) indicating its reception environment to the base station. In other words, it is disclosed that the mobile device estimates a variation of the amplitude and phase in a channel and generates CQI by using a pilot signal transmitted from the base station and feeds back the generated CQI to the base station, and the base station performs an AMC control based on the CQI fed back from the mobile device.
In this case, a pilot signal includes a common pilot signal transmitted independently of the presence or absence of data for each mobile device assigned to a plurality of subcarriers and a dedicated pilot signal transmitted along with data transmission from a base station to a specific mobile device. It is disclosed that the mobile device generates CQI for each subcarrier by using the common pilot signal and feeds back the generated CQIs to the base station and that the mobile device selects subcarriers having good reception environment among the generated CQIs and feeds back the selected subcarriers to the base station.
On the other hand, there is proposed a technique for applying a dedicated pilot signal transmitted by the base station in addition to data for the mobile device to beam forming transmitted by different directivity for each mobile device. Beam forming is a technique that a base station forms a directional beam for a mobile device that is a transmission destination by using a plurality of antennas and adjusts directivity to increase the received power of the mobile device.
It is disclosed that a mobile device generates CQI by using a dedicated pilot signal associated with data for itself when applying the beam forming.
Moreover, it is disclosed that a base station previously transmits a dedicated pilot signal inherent in a mobile device to all subcarriers to first assign packet data to a radio resource and feeds back CQI generated on the basis of the dedicated pilot signal for each mobile device.
Furthermore, it is disclosed that a mobile device gathers, for a long time, offsets that indicate a difference between CQI generated by using a dedicated pilot signal associated with data for itself and CQI generated by using a common pilot signal to compute an average value and generates CQI that indicates an instantaneous reception environment on the basis of the computed average value and CQI generated by using an instantaneous common pilot signal.